The day their kids' report cards came home last month, Reid Pedersen and his wife, Laureen, sat up in bed having a worried conversation. Their eight-year-old daughter was the same high-achieving child she had always been. But their five-year-old son, who was taking longer to grasp concepts, and with less confidence – was he where he should be?

As most parents of two or more kids know, the time and resources you spend trying to boost their brainpower can differ drastically.

His daughter was always in front of Sesame Street, learning numbers with the Count and having her ABCs drilled into her through repetition with Big Bird and the gang. Looking back, he wonders about his son. "I don't think he's ever seen that," Mr. Pedersen says of the television show.

It's a familiar story for anyone with two kids. The time spent with flip cards and practising letters and doing puzzles to learn shapes or animals drops precipitously from your first child to your second.

Just as familiar is the temptation to see the differences between siblings as the result of birth order. We all know the stereotype of go-getting firstborns and laid-back second children. It's a notion that has been with us for nearly a century, from the theories of Sigmund Freud to countless online quizzes that promise to divine your birth order based on your personality.

But a handful of recent studies, including one recently published in the Journal of Human Resources conducted by economists at the University of Edinburgh, have shed new light on the birth-order effect, which they suggest is only a myth – with one minor exception.

A THEORY IS BORN

There is plenty of history to support the notion that personality is bred in the bone through birth order. For example, look no further than old English laws of primogeniture, which saw first-born children enjoying the entitlement of their superiority by getting pride of place in inheritance.

When it is time to crown a king or queen, first dibs usually go to the eldest child.

"In all cultures and societies people have had rules for the first-born taking over, so there's this idea that the first-born is more responsible," says Rodica Damian, an assistant professor of social psychology at the University of Houston.

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In the early days of psychoanalysis, Freud, a first-born child, clashed with Alfred Adler, a middle child, over what kind of psychoses might be typical of people based on birth order.

Freud thought first-borns were the most well-adjusted. Adler thought first-borns are neurotic because they feel dethroned. Last-borns are always ignored. The middle child is best adjusted, according to Adler.

"They each thought their respective birth order had the least neuroses," Damian says.

In the years since psychoanalysis fell out of favour, a new theory based on evolutionary thinking has become dominant.

"Each child fights for an evolutionary niche to get the most attention and resources of parents," explains Damian.

First-borns try to live up to their parents' expectations so that's why they tend to be more responsible, more parent-pleasing and more dominant.

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"The second-born has to fill a different niche to get attention … so they have to be more funny, more creative, more rebellious, more sociable," Damian says.

It is clearly a subject of perennial fascination, with nearly 3.2 million birth-order citations indexed on Google Scholar.

"There's been research for about 100 years on this and it's all been very conflicting," says Damian, who co-authored the largest-ever birth-order study, in 2015. "We had this huge data set, so we thought it was the perfect opportunity to look at this question and hopefully help settle the debate once and for all."

THEY'VE GOT PERSONALITY

The evolutionary theory that is currently in vogue has launched countless parenting magazine articles that purport to help parents understand kids' personalities based on birth order with the same clear-sighted certainty as an astrologer who can tell the difference between a know-it-all Capricorn and an indecisive Libra.

According to the theory, first-born children are better leaders, more neurotic, more responsible, less impulsive, while later-born children are more sociable, more extroverted, more self-confident and more agreeable.

But in the 2015 study by Damian and colleagues, birth order's effect on personality was found to be nil.

The study looked at data collected by the American Institutes for Research of 377,000 high-school students across the United States, including their personality measures and IQs, a huge opportunity to overcome one of the main flaws of many previous studies – very small sample sizes.

"The total sample is bigger than all the previous study samples put together," Damian says.

Such a large number allowed researchers to examine nearly every type of variation in family structure imaginable: half-siblings, only children, adopted kids.

"Because the data is so big it allows you to get a reliable estimate of really, really special groups," Damian says.

And the overall effect on personality that researchers were able to detect? In many cases, it was zero. On many personality measures, findings ran contradictory to the current theory. The highest correlation was a minuscule 0.08. For most others, it was a measly correlation of 0.02. That's equal to one-50th of a standard deviation. In order to see visible differences in the real world you need at least one full standard deviation.

"The most important thing is that the direction of the effects does not follow that niche hypothesis," Damian says.

In other words, the study found no evidence to support our current understanding of birth order's effect on personality – because there is no such effect.

As most parents of two or more kids know, the time and resources you spend trying to boost their brainpower can differ drastically.

Getty Images/iStockphoto

IQ

This is where things get interesting. While studies have never agreed on the relationship between birth order and personality, with some saying birth order predicts all the traits we think it would, others saying it predicts only some of those traits and still more saying it has zero effect, researchers have all noticed one consistent finding: First-borns are smarter.

Before you go rubbing that in to your younger brother, however, well – don't get too far ahead of yourself, smart guy.

In Damian's study, first-borns were smarter on average by one underwhelming IQ point (less than the margin of error on a standard IQ test).

Another large study conducted by researchers at the University of Leipzig and also published in 2015 made a similar conclusion: First-borns are smarter, but just barely.

After analyzing the data looking for effects on personality both within families and across families, the study found no impact on personality, and only a slight effect on IQ.

"We are really sure that this effect exists but it's super-small," says Julia Rohrer, co-author of the study, which looked at data from 20,000 people from Germany, Britain and the United States.

In fact, first-borns were found to be between one and three IQ points higher on average.

But that doesn't mean first kids will always be smarter, Rohrer cautions.

After all, averages across large populations can't be applied to individuals.

"We found that in 60 per cent of the cases the first-born was smarter, but in 40 per cent of the cases, the second-born was smarter," she says.

FINALLY, IQ EXPLAINED

If researchers consistently find that, at least on average, first-born children tend to score higher on IQ tests, the question is, why?

Look no further than the Pedersens, or likely your own experience if you have more than one child.

Researchers call it the confluence theory. Put simply, parents put more time and resources into their first child, especially more time talking to them, which is why first-borns tend to do better on the verbal-ability section of IQ tests.

A study published in November in the Journal of Human Resources looked at nearly 5,000 children who were assessed every two years, from the time they were born to age 14.

Researchers found that parents put in less time reading to second kids and subsequent children and less time on other forms of mental stimulation such as doing crafts and playing musical instruments.

"As you have more kids you have less time and less attention," says Ana Nuevo-Chiquero, a lecturer in the School of Economics at the University of Edinburgh, who led the study.

BUT, BUT, BUT

"But explain this to me," I said to Damian. "I hear everything that you are saying and rationally it makes sense, but why do I look at my two children and see perfect stereotypes of birth-order personalities?"

She laughed, and mentioned a commentary she and her fellow researchers published after their study about exactly this topic.

"We believe birth order will persist as a zombie theory for a very long time," she says. "Despite all the science behind it, people have very, very strong feelings about it."

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